Method of using radioactive tracers



United States Patent O M 3,130,314 METHQD OF USING RADIOACTIVE TRACERSAlan Beer-bower and John L. Murray, Westfield, N.J., assignors to EssoResearch and Engineering Company, a corporation of Delaware N Drawing.Filed Nev. 23, 1959, Ser. No. 854,530 11 Claims. (Cl. 2501tl6) Thepresent invention relates to an improved method of using radioactivetracers for the purpose of studying mixing, fluid flow, leakage andsimilar problems, particularly in respect to the processing of petroleumhydro carbons in oil refineries and similar installations. Moreparticularly, the improved tracer technique herein devised relates tothe development of a practical, cheap, and improved method of usingtracers that have a relatively short half life.

The use of radioactive tracer elements in the determination of rates offlow, mixing, inventory, leakage, and, in general, as an inspection toolin the chemical and petroleum refining industry is well-known. Thishighly useful technique has, as in the past, been attendant with severalshortcomings; for example, some of the radioactive isotopes possessextremely long half lives and, particnlarly in instances where thetracer isotopes are to be directly injected into the process streams,reaction vessels and the like, the use of isotopes having a long halflife, for example, strontium 90 or cobalt 60 presents contaminationproblems because in most instances, at least small amounts of theradioactive materials finally appear in the products of reaction. Thisnecessitates safety precautions in the storage and handling of suchproducts or it requires the decontamination of the products prior totheir sale on the open market or prior to their usage in otherprocesses, particularly in installations where safety precautions havenot or cannot be instituted and main tained.

Where a direct injection of the isotope tracer elements is to befollowed, in most cases it becomes a general necessity to employisotopes having relatively short half lives; i.e., of the order of 36hours or less. It is recognized, moreover, that from one standpoint,namely that of safety, the shorter the half life of the isotopeemployed, the less expensive storage or decontamination facilities thatare required. On the other hand, while isotope having a half life of theorder of a few minutes can be desirably employed in some instances,oftentimes the purely physical requirements of manipulation, injection,size of equipment, and the process through which the progress of thetracer is to be followed, necessitate the use of tracers having halflives of at least several hours. Almost invariably when using suchtracers, recycle streams, exit product lines, storage tanks, surgetanks, and other various pieces of equipment connected with theprocessing become contaminated, and it is not possible to select aparticular radioactive isotope so that the contamination problem andattendant radiation hazard is eliminated or obviated at the exact pointand time at which the isotope finally emerges with the product from theprocess. In general and for practical purposes, most radioactive tracersemployed can be considered as radioactively dangerous for a period ofroughly ten times their half life. It is therefore desirable inemploying a radioactive isotope of a particular element, for tracerstudies, to select one which will have a sufficiently long totalradioactive life that a thorough and complete inspection of the systemcan be accomplished while, at the same time, avoiding radiation hazardsdue to the contamination of the final product for any great period oftime after that product is finally withdrawn from the system understudy.

The present invention seeks to accomplish these ends by shortening thetime for preparing the isotope tracer 3,139,314 Patented Apr. 21, 1964sample for injection and, thus, making possible the use of a tracer witha correspondingly shorter half life than Would otherwise be the case.For example, British Patent 795,028 attempted to solve this problem bydispersing isotope in an aqueous solution. This aqueous solution wasthen admixed with oil and alcohol and another solvent in order toproduce either a true solution or at least a homogeneous mixturesuitable for injection into a system that was primarily water immiscibleand that would not ordinarily distribute an aqueous solution in auniform manner. Prior to the introduction of the tracer, the alcohol andthe solvent were removed by either distillation or evaporation beforeinjection of the solution into the system. Both of these proceduresrequire additional time, thus cutting down on the remaining time thatthe isotope will be useful in the subsequent tracer studies. This is areal problem, particularly in cases where sodium 24, aluminum 28, andiodine 132 are employed, because their half lives are respectively 15hours, 2.3 hours, and 2.33 hours. Similarly, Te 99 has a half life ofapproximately six hours.

The present invention has obviated the necessity for employing suchcumbersome and time-consuming techniques in preparing an isotope samplefor injection into a system for tracer studies. The invention isparticularly useful for a system whose predominant components arewater-immiscible such as hydrocarbon streams, crude oil processing,alkylation processes, fluid coking operation or, in fact, any systememploying petroleum oils in the liquid phase.

The novel process eliminates distillation and evaporation steps and isable to accomplish the solubilizing or the preparation of homogeneousmixtures for injection in tracer studies in a minimum of time and isthus able to employ isotope tracers of shorter half lives than hasheretofore been thought possible to accomplish the same ends. For mostrefinery inspection purposes, the ideal half life of a tracer is about36 hours or less, preferably about one 8-hour shift. In this way thedisposal or safety precaution problem is practically non-existent. Thesolution of the problem involved lies in the use of solubilizing orsurface active solubilizing agents. The extremely powerful emulsifyingor surface active agents which are useful in conditioning the aqueoussolutions of water soluble salts of the radioactive isotopes employedmay be any of those commonly available to the trade today; The followingare examples of these agents and are of the type of surface activesolubilizing agents that are particularly useful in practicing theherein described novel process: Sodium mahogany sulfonates, sodiumkeryl-benzene sulfonates, fatty alcohol sodium sulfates, amine salts ofthese acids, sorbitol oleates, alkyl-phenol ethylene oxide condensationproducts, metallic naphthenates, fatty alcohol-ethylene oxidecondensation products, alkali or amine salts of alkyl phosphoric acid,partial esters, acetylenic alcohols. Additional specific solubilizingagents are: An emulsifier concentrate of sodium mahogany sulfonateconcentrate admixed with sodium naphthenate, diethylene glycol andwater, and the potassium soap of tall oil admixed with isopropyl alcoholand water and containing sodium sulfonate.

One source of radioactive tracer materials which have relative shorthalf lives and are suitable for the practice of the present invention,is described in the article entitled Development of Methods for theProduction of Certain Short-Lived Radioistotopes, by L. G. Stang, Jr.,et al., UNESCO/NS/RIC/ 190, and appearing in Radioisotopes in ScientificResearch, vol. 1, Pergamon Press, London, N.Y., and Paris, 1958, pp.50-70. A system of mother-daughter pairs takes care of the problem oftransportation. In short, a generator or milker is i supplied from whicha solution of the daughter isotope Mother Daughter Stable Product To 132I 132 Xe 132 Ba 140 La 140 Ce 140 Us 137 Ba 137 La 137 lVIg 28 Al 28 Si28 Mo 99 Te 99 Ru 99 On the other hand, many elements do not lendthemselves to this type of transportation and must be either shipped asrapidly as possible from the nearest nuclear reactor or prepared on thespot by bombardment with neutrons, deuterons, etc. from an electricalaccelerator. Such isotopes useful in the present process are Na 24, Mn56, Br 82, As 76, Ga 72, Cu 64, and K 42. These are seldom carrier freeand often contain over 99.9% of inactive isotopes. The chloride,carbonate, nitrate or any other water soluble salt of the radioisotopeis prepared in an aqueous solution depending upon the chemical nature ofthe particular element. Relatively small amounts of radioactive metalsalt in water may be employed.

In one specific example about 0.07 microgram of the salt Na 24 nitrateplus 25 grams of carrier Na 23 nitrate in about 10 grams of water wasmixed with about 500 grams of an emulsifier concentrate and stirred forabout one minute. The activity was 200 millicuries. A light naphtha inthe amount of about 500 cc. was then added and stirred for an additionalminute. The final mixture was clear and homogeneous and placed in apressure bomb and forced by nitrogen pressure into the hot pitch feedline leading to a fluid coker operation. The usual instruments fordetecting gamma radiation through metal were employed for recording thepath of the hot pitch feed and a flow pattern, residence time and thelike was determined based upon the gamma radiations recorded by theinstruments. The emulsifier concentrate employed contained the followingingredients in the approximate percentages shown:

52% Na mahogany white oil sulfonate 21% naphthenic lubricating oil of300 SSU 100 F. 15% Na naphthenate 5% diethylene glycol 7% water In placeof the light naphtha or Varsol, a small amount of the feedstock itself(if suitable) may be mixed with the solubilized aqueous tracer solutionby stirring or violent agitation, and this concentrate may then beinjected into the system for the conducting of tracer studies. Sodium 24nitrate having a half life of about hours was employed in the particularstudy undertaken so that at the end of about one weeks time the naphthaproducts of the reaction were so low in radioactive contamination as tobe of no potential hazard to human beings.

In a second example, the liquid from an I 132 milker was used. Thisconsisted of about 5 10 micrograms of NaI 132, about 2.5 grams of sodiumacetate and about 0.5 gram of acetic acid in about 40 cc. of water. Thiswas solubilized in about 17 lbs. of lead naphthenate and about 17 lbs.of lubricating oil to form a concentrate, which in turn was added to anunmixed 17,000 gal. batch of gear lubricant. Mixing was then stated, anda Geiger counter used to measure the activity of the batch at variouslevels. It was possible to detect differences until at about 20 minutesthe 'batch proved to be uniform. The

4 I 132, with its short half life of 2.33 hours, was decayed toundetectability in about 24 hours.

The use of surface active agents involved a very short period of time inpreparing the radioactive concentrate for injection into the hydrocarbonsystem so that it was possible to employ a relatively short half lifeisotope and, at the same time, avoid any major amount of decay of theactivity before the test was under way and completed. The shortening ofthe time required in the preparation operation and the obtaining of auniform distribution of the isotope in the system under inspection weretwo major advantages in the use of the novel technique employed.

In general, the radioisotope is but a minor constituent of the mixture.The inert carrier may be present in substantial amounts or totallyabsent, depending on the method of preparation of the isotope. It willrange from 0 to 5% of the final mixture. Reagents used in milking ordissolving the isotope may amount to from 0 to 2%. The water must beadequate to dissolve the carrier and/ or reagents, and is usually from 2to 15% of the final mixture. to 50 times the amount of water plus salts;10% to of the final preparation injected may be this material. Diluentmay not be required but, if used, the amount will vary from 0% to 75%,the exact quantity being a function of solubilizing agent Viscosity andthe viscosity of the stream into which the injection is to be made.

Having now thus fully described and illustrated the nature of theinvention, what is desired to be secured by Letters Patent is:

1. An improved method of employing radioactive tracers which comprises,admixing a surface active solubilizing agent with an aqueous solution ofa water soluble salt of a radioactive element having a relatively shorthalf life, injecting said mixture into a water-immiscible hydrocarbonand determining by use of radiation sensitive instruments the locationof the radioactive material in said hydrocarbon.

2. A process as in claim 1 wherein the aqueous mixture containing thesurface active agent is mixed with a portion of the hydrocarbons intowhich it is to be introduced prior to introducing the admixture intosaid hydrocarbon.

3. A process as in claim 1 wherein the radioactive element has a halflife of less than about 36 hours.

4. A process as in claim 1 wherein the radioactive element is Na 24.

5. A process as in claim 1 wherein the radioactive element is I 132.

6. A process as in claim 1 wherein the surface active agent is anorganic sulfonate.

7. A process as in claim 1 wherein the surface active agent is a mixtureof sodium mahogany white oil sulfonate, sodium naphthenate, lightlubricating oil, diethylene glycol and water.

8. A process as in claim 1 wherein the surface active agent is a mixtureof sodium sulfonate, salts of long chain fatty acids, potassium soap oftall oil, isopropyl alcohol and water.

9. A process as in claim 4 wherein the radioactive element is sodium 24in the form of its nitrate.

10. A process as in claim 5 wherein the radioactive element is I 132 inthe form of its sodium salt.

11. An improved method of employing short life radioactive tracers,which method comprises admixing a surface active solubilizing agent withan aqueous solution of a water soluble salt of a radioactive elementhaving a half life of less than about 36 hours, the amount of thesurface active agent being from 5 to 50 times the amount of the aqueoussolution, and injecting said mixture into a liquid petroleum hydrocarbonstream whereby the location of said radioactive element may bedetermined with radiation instruments.

(References on following page) The solubilizing agent is at least fivetimes and up References Cited in the file of this patent UNITED STATESPATENTS Ferris Apr. 6, 1943 Herzog June 22, 1948 De Forrest June 7, 1949Stokeley et a1. Aug. 16, 1949 Linderman June 15, 1954 Juterbock et alMay 1, 1956 Fries May 20, 1958 OTHER REFERENCES Gore et al.: RadioactiveTracer Techniques, Journal of Petroleum Technology, September 1956, pp.1216.

1. AN IMPROVED METHOD OF EMPLOYING RADIOACTIVE TRACERS WHICH COMPRISES,ADMIXING A SURFACE ACTIVE SOLUBILIZING AGENT WITH AN AQUEOUS SOLUTION OFA WATER SOLUBLE SALT OF A RADIOACTIVE ELEMENT HAVING A RELATIVELY SHORTHALF LIFE, INJECTING SAID MIXTURE INTO A WATER-IMMISCIBLE HYDROCARBONAND DETERMINING BY USE OF RADIATION SENSITIVE INSTRUMENTS THE LOCATIONOF THE RADIOACTIVE MATERIAL IN SAID HYDROCARBON.